1. Field of the Invention
This invention relates in general to jacks of the piston-cylinder type and more particularly to such jacks provided with drain valves.
2. Description of the Prior Art
Piston type jacks are well known in the art. Such jacks are most often provided with an internal hydraulic fluid check valve. Of those which permit entrance of fluid into the cylinder from an external source, outward flow of the hydraulic fluid is usually prevented. A common problem with such units is that it is often desirous to deactivate the jack immediately upon removal of the workpiece to be compressed or upon retraction of the jack; and to remove fluid from the cylinder after compression to replace the piston to its original position. To remove the fluid the hydraulic valve may be removed or a manual stopcock is provided. Drain valves which are either activated or deactivated by overcoming a compressor spring, as typified by U.S. Pat. No. 3,140,722 issued to C. G. Gordon and U.S. Pat. No. 3,246,664, are generally used to bypass hydraulic fluid wen the pressure within the system exceeds a predetermined value. While such valves are acceptable for the purpose for which they are intended, such valves are not usable directly with a workpiece to maintain the integrity of the valve when directly in contact with the workpiece; to drain the fluid from the cylinder by simply compressing the piston; and then to again activate the jack by contact with the workpiece.